SELECTING POWER OF THE R007 S 79 



IV. We are thus brought to the consideration of the alleged selecting 

 power of the roots, which, if rightly attributed to them, must be con- 

 sidered as one of the most important functions of which they are pos- 

 sessed. It is a function, however, the existence of which is disputed by 

 many eminent physiologists. But as the adoption or rejection of it will 

 materially influence our reasonings, as well as our theoretical views, in 

 regard to some of the most vital processes of vegetation, — it will be pro- 

 per to weigh carefully the evidence on which this power is assigned to 

 the roots of plants. 



1°. The leaves, as we shall hereafter see, possess in a high degree 

 the power of selecting from the atmosphere one or more gaseous sub- 

 stances, leaving the nitrogen, chiefly, unchanged in bulk. The absorp 

 tion of carbonic acid and the diminution of the oxygen in the experi 

 ments above described, appear to be analogous effects, and would seem 

 to imply in the roots the existence of a similar power. 



2°. Dr. Daubeny found that pelargoniums, barley {hordeum vulgare)^ 

 and the winged pea {lotus tetragonolohus), though made to grow in a 

 soil containing much strontia,* appeared to absorb none of this earth, foi 

 none was found in the ash left by the stem and roots of the plant when 

 burned. In like manner De Saussure observed that polygonum persi- 

 caria refused to absorb acetate of lime from the soil, though it freely took 

 up common salt. — [Lindley's Theory of Horticulture, p. 19.] 



3°. Plants of different species, growing in the same soil, leave, when 

 burned, an ash which in every case contains either different substances, 

 or the same substances in unlike proportions. Thus if a bean and a 

 grain of wheat be grown side by side, the stem of the plant from the lat- 

 ter seed will be found to contain silica, from the former none.f 



4°. But the same plant grown in soils unlike in character and com- 

 position, contains always — if they are present in the soil at all— very 

 nearly the same kindj of earthy matters in nearly the same proportion. 

 Thus the stalks of corn plants, of the grasses, of the bamboo, and of many 

 others, always contain silica, in whatever soil they grow, or at least are 

 capable of growing with any degree of luxuriance. 



With the view of testing this point, Lampadius prepared five square 

 ])atches of ground, manured them with equal quantities of a mixture of 

 horse and cow dung, sowed them with equal measures of the same 

 wheat, and on four of these patches strewed respectively five pounds of 

 finely powdered quariz (siliceous sand), of chalk, of alumina, and of 

 carbonate of magnesia, and left one undressed. The produce of seed 

 from each, in the above order, weighed 24i, 28|, 26i, 21i, and 20 ounces 

 respectively. The grain, chaff", and straw, from each of the patches 

 left nearly the same quantity of ash — the weights varying only from 3-7 

 to 4-08 per cent., and the roots and chaff'being richest in inorganic mat- 

 ter. The relative proportions of silica, alumina, lime, and magnesia, 



* Watered with a solution of nitrate of strontia. Strontia is an earthy substance resem 

 bling lime, which is found in certain rocks and mineral veins, but which has not hitherto been 

 observed in the ashes of plants. 



t It is not strictly correct that the bean will absorb no silica, but the quantity it will take up 

 will be only one-thirteenth of that taken up by the wheat plant— the per centage of silica in 

 the ash of bean straw being, according to Sprengei, only 22, while in wheat straw it is 287 

 per cent. Pea straw contains four times as much as that of the bean, or 996 per cent. 



X For more precise information on this point, see the subsequent lectures, " On the inor- 

 ganic constituents of plants," (Part II.) 



